4 Answers
4

It's not unprofessional to decline a job offer. It is unprofessional, though, to look for a job that you have no intention of taking regardless of the offer.

Now, realistically, it's unlikely that anyone would know whether you're interviewing just for practice and that you'll go on to graduate school regardless of the offer or whether you're honestly considering your options. You're wasting the time of everyone you meet with and forcing them to do a lot of work that they wouldn't do if you were forthright with them. But as long as you don't admit to your motives, it's unlikely that it would negatively affect you.

If you turn down a position at a particular company, that may affect your ability to get a job at that company in the future. Policies depend, of course, on the company along with the job that you're applying for now vs. the job you turned down and how much time has passed. It's unlikely to disqualify you but a company may be hesitant to hire you now if you thought it was a poor fit in the past-- that might lead the company to believe that you're only settling for them now and that you'll be moving on to a different position shortly. Applying for a different position years later will be less of a concern than if you apply again in a few months for the same position because you're finding graduate school less interesting than you expected.

Most people looking for jobs will apply at more than one place, so it's always possible and normal to get more than one job offer, and obviously one of them must be refused. That's normal and professional. Or they might make an offer with conditions that you don't want to accept (like half the salary you're worth). Refusing is normal and professional. Just don't apply for jobs that you have no intention to take.
– gnasher729Jul 9 '14 at 10:54

If this is just an experiment to collect data, then you are wasting peoples time because you have no plan to accept the job.

But run this past your placement office at the university if you need help deciding to work now or go to graduate school full time. They can tell you what companies may be offering for people with your degree. They can also discuss typical benefits levels from the companies they work with. These benefits could also include help for graduate school tuition. They may be able to set up an appointment with a company that can answer some of your questions regarding pay and benefits for a typical position you might be qualified for.

Again only go through this process if you will be using the information to make a career path decision.

If you go through the charade and are offered the job you will have to either tell the truth, and risk getting blacklisted by that company. Or lie and get away with it. I suggest only going though the process if you really considering the offer.

Also bear in mind that it's not too uncommon for large companies to have a policy where they will not interview anyone that has already been through the process in the last X years. Don't harm your chances of getting a job when you do need it.
– Fiona - myaccessible.websiteNov 13 '13 at 16:00

Probably not as much as you think, if you are coming out of school/university most companies will need to train you to do the job they need.

How an interview process actually goes?

Your school or university possibly has some extra curricular activities which focus on this. There are also organizations which can help you with this, as you don't state your country I can't point you in any directions.

How much chance there is of me being hired by a good company?

About 0% chance, unless they are well staffed and a manager looks kindly on you most good companies have people beating down their door.

Will this be considered unprofessional:

If I refused job offer after getting it?

No, whatever somebody might say it's not unprofessional to refuse a job offer. It however is unkind to waste somebody's time, just as you would find it annoying if somebody wasted your time.

If yes how much impact it will have on future career?

Almost certainly none.

EDIT: After a discussion in the comments I realized that the best practice would really be to do interviews for a part-time job next to your future studies.

You get the answers to the things I left our, such as organizations who can help you learn interviewing skills, with some simple searches with your favourite search engine.

@JoeStrazzere I do mention that it is unkind to waste anybody's time, and interviewing if you have no intention of taking the job - lying as you put it - is wasting somebody's time. Besides of course he is really available for work, unless his parents are filthy rich everybody needs some form of income.
– Daniël W. CromptonNov 12 '13 at 22:44